The overall goal of this project is to understand the roles of hepatocyte nuclear factor-13 (HNF-1(3)in kidney-specific gene expression, renal cell differentiation, and kidney organogenesis. HNF-1(3belongs to a family of homeodomain-containing transcription factors that regulate tissue-specific gene expression in the kidney, liver, pancreas, and other organs. Humans with mutations of HNF-13 develop maturity-onset diabetes of the young type 5 (MODY5) and congenital cystic abnormalities of the kidney. Transgenic mice expressing mutant HNF-13 under the control of a kidney-specific promoter develop kidney cysts and renal failure, which is similar to the phenotype of humans with MODY5. Similarly, kidney-specific deletion of HNF- 1(3 using Cre/loxP recombination results in renal cyst formation. HNF-1{3mutant mice show decreased expression of Pkhdl, the gene mutated in autosomal recessive polycystic kidney disease (ARPKD), and HNF-13 directly regulates the Pkhdl promoter. These studies identify Pkhdl as a novel gene target of HNF- 13 in the kidney. They establish a previously unrecognized link between two renal cystic diseases, MODY5 and ARPKD, and suggest that the mechanism of cyst formation in humans with MODY5 involves down- regulation of PKHD1 gene expression. To test this hypothesis and to further define the functions of HNF-13 in the kidney, we will complete the following specific aims: 1. Define the roles of coactivators and histone acetylation in the regulation of Pkhdl gene transcription. 2. Determine how the activity of HNF-13 is reciprocally regulated by SUMO ligases and ubiquitin ligases. 3. Determine how HNF-13 regulates the tissue-specific expression of Pkhdl in the kidney and liver. 4. Elucidate the molecular pathogenesis of the kidney and genitourinary tract abnormalities caused by mutations of HNF-13. The proposed studies will utilize genetically-modified mice to define in vivo expression patterns and disease phenotypes and biochemical studies to elucidate molecular mechanisms. Understanding the functions of HNF-13 and the regulation of Pkhdl gene transcription will provide insights into the pathogenesis of congenital kidney abnormalities and ARPKD,which is one of the most common genetic causes of renal failure in infants and children.